Expanders for the lower teeth
Sometimes it is desirable to widen the teeth in the lower jaw. The appliance shown below is about to be cemented into the mouth - it is known as a Bihelix because of the two coils bent into the wire.
Expanding the lower teeth is a much more uncommon procedure.
Expanders for the upper teeth
This picture shows a recently cemented upper arch expander - known as a Quadhelix because of the four spring coils bent into the wire.
This appliance is commonly used to widen the upper teeth to fit around the lower teeth.
Upper arch expansion is usually done to correct cross-bites of the back teeth and at the same time may be used to create some space to align crowded front teeth.